11,219 research outputs found
Lightweight multiple output converter development
A high frequency, multiple output power conditioner was developed and breadboarded using an eight-stage capacitor diode voltage multiplier to provide +1200 Vdc, and a three-stage for -350 Vdc. In addition, two rectifier bridges were capacitively coupled to the eight-stage multiplier to obtain 0.5 and 0.65 a dc constant current outputs referenced to +1200 Vdc. Total power was 120 watts, with an overall efficiency of 85 percent at the 80 kHz operating frequency. All outputs were regulated to three percent or better, with complete short circuit protection. The power conditioner component weight and efficiency were compared to the equivalent four outputs of the 10 kHz conditioner for the 8 cm ion engine. Weight reduction for the four outputs was 557 grams; extrapolated in the same ratio to all nine outputs, it would be 1100 to 1400 grams
Multigrid solution of compressible turbulent flow on unstructured meshes using a two-equation model
The system of equations consisting of the full Navier-Stokes equations and two turbulence equations was solved for in the steady state using a multigrid strategy on unstructured meshes. The flow equations and turbulence equations are solved in a loosely coupled manner. The flow equations are advanced in time using a multistage Runge-Kutta time stepping scheme with a stability bound local time step, while the turbulence equations are advanced in a point-implicit scheme with a time step which guarantees stability and positively. Low Reynolds number modifications to the original two equation model are incorporated in a manner which results in well behaved equations for arbitrarily small wall distances. A variety of aerodynamic flows are solved for, initializing all quantities with uniform freestream values, and resulting in rapid and uniform convergence rates for the flow and turbulence equations
Linear feedback control of transient energy growth and control performance limitations in subcritical plane Poiseuille flow
Suppression of the transient energy growth in subcritical plane Poiseuille
flow via feedback control is addressed. It is assumed that the time derivative
of any of the velocity components can be imposed at the walls as control input,
and that full-state information is available. We show that it is impossible to
design a linear state-feedback controller that leads to a closed-loop flow
system without transient energy growth.
In a subsequent step, full-state feedback controllers -- directly targeting
the transient growth mechanism -- are designed, using a procedure based on a
Linear Matrix Inequalities approach. The performance of such controllers is
analyzed first in the linear case, where comparison to previously proposed
linear-quadratic optimal controllers is made; further, transition thresholds
are evaluated via Direct Numerical Simulations of the controlled
three-dimensional Poiseuille flow against different initial conditions of
physical interest, employing different velocity components as wall actuation.
The present controllers are effective in increasing the transition thresholds
in closed loop, with varying degree of performance depending on the initial
condition and the actuation component employed
E-ELT constraints on runaway dilaton scenarios
We use a combination of simulated cosmological probes and astrophysical tests
of the stability of the fine-structure constant , as expected from the
forthcoming European Extremely Large Telescope (E-ELT), to constrain the class
of string-inspired runaway dilaton models of Damour, Piazza and Veneziano. We
consider three different scenarios for the dark sector couplings in the model
and discuss the observational differences between them. We improve previously
existing analyses investigating in detail the degeneracies between the
parameters ruling the coupling of the dilaton field to the other components of
the universe, and studying how the constraints on these parameters change for
different fiducial cosmologies. We find that if the couplings are small (e.g.,
) these degeneracies strongly affect the constraining
power of future data, while if they are sufficiently large (e.g.,
, as in agreement with current
constraints) the degeneracies can be partially broken. We show that E-ELT will
be able to probe some of this additional parameter space.Comment: 16 pages, 8 figures. Updated version matching the one accepted by
JCA
Multigrid solution of the Navier-Stokes equations on triangular meshes
A Navier-Stokes algorithm for use on unstructured triangular meshes is presented. Spatial discretization of the governing equations is achieved using a finite element Galerkin approximation, which can be shown to be equivalent to a finite volume approximation for regular equilateral triangular meshes. Integration steady-state is performed using a multistage time-stepping scheme, and convergence is accelerated by means of implicit residual smoothing and an unstructured multigrid algorithm. Directional scaling of the artificial dissipation and the implicit residual smoothing operator is achieved for unstructured meshes by considering local mesh stretching vectors at each point. The accuracy of the scheme for highly stretched triangular meshes is validated by comparing computed flat-plate laminar boundary layer results with the well known similarity solution, and by comparing laminar airfoil results with those obtained from various well-established structured quadrilateral-mesh codes. The convergence efficiency of the present method is also shown to be competitive with those demonstrated by structured quadrilateral-mesh algorithms
Combined Relativistic and static analysis for all Delta B=2 operators
We analyse matrix elements of Delta B=2 operators by combining QCD results
with the ones obtained in the static limit of HQET. The matching of all the QCD
operators to HQET is made at NLO order. To do that we have to include the
anomalous dimension matrix up to two loops, both in QCD and HQET, and the one
loop matching for all the Delta B=2 operators. The matrix elements of these
operators are relevant for the prediction of the B-\bar B mixing, B_s meson
width difference and supersymmetric effects in Delta B=2 transitions.Comment: 3 pages, 1 figure. Lattice2001(heavyquark
Constraining spatial variations of the fine-structure constant in symmetron models
We introduce a methodology to test models with spatial variations of the
fine-structure constant , based on the calculation of the angular power
spectrum of these measurements. This methodology enables comparisons of
observations and theoretical models through their predictions on the statistics
of the variation. Here we apply it to the case of symmetron models. We
find no indications of deviations from the standard behavior, with current data
providing an upper limit to the strength of the symmetron coupling to gravity
() when this is the only free parameter, and not able to
constrain the model when also the symmetry breaking scale factor is
free to vary.Comment: Phys. Lett. B (in press
Non-perturbative renormalisation of four fermion operators and B-bar B mixing with Wilson fermions
We present new results for the renormalisation and subtraction constants for
the four fermion Delta F=2 operators, computed non-perturbatively in the RI-MOM
scheme (in the Landau gauge). From our preliminary analysis of the lattice data
at beta=6.45, for the B-bar B mixing bag-parameter we obtain B_B^{RGI} =
1.46(7)(1).Comment: 3 pages (4 figures), Lattice2002(heavyquark
An Examination of the Impact of Attending an Electronic Technology Rich Program on the Teaching Styles of Senior Student Teachers
The purpose of this study was to determine whether attending an electronic technology rich program had an impact on the teaching styles of senior student teachers. It is clear from this research that senior student teachers have benefits from attending such a program. Three research questions were used to guide this study: what ways teachers are currently using technology; how are teachers in this study using technology in their planning of the learning experience; how teachers\u27 technology attitudes, understandings and skills change over time. A web based survey tool, which was delivered electronically to senior student teachers both in elementary and secondary environments. The survey instrument consisted of 34 questions and was comprised of four parts and designed to have two measurement properties, labeled categories (nominal scale) and variable differences (interval scale). Collected data showed a willingness on the part of the senior student teachers to use electronic technologies, as well as a possessing a strong comfort level using multiple electronic technologies and software applications such as Word and PowerPoint. While the two software applications least likely to be used by senior student teachers were spreadsheet applications such as Excel and databas applications like Access. The infusion of electronic technologies into lessons and curriculum was quite strong but data showed varied results as to how senior student teachers view the effect of such infusion. Nearly all respondents agreed that PowerPoint is an effective tool in both lesson planning and presentation by both students and teachers alike. Recommendations for future research include should take place on either an annual of semi-annual basis, in addition to extending the participant audience to include junior pre-service teaching candidates
Image Repair in NCAA Division I Athletics
NCAA Division I universities are constantly under a threat of image loss due to both internal and external factors. The purpose of this study was to examine past image repair campaigns executed after a crisis by the following NCAA Division I member universities: Ohio State University, Louisville University, and the University of Maryland. A problem-oriented case study was completed on the media coverage of these situations, which evaluated the effectiveness of the strategies chosen by each respective university. Results revealed the best practices for public relations practitioners to repair and maintain a positive image in the collegiate landscape
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